1. Field of the Invention
The present invention relates to an exhaust heat exchanger system designed to recover heat from exhaust gas emitted from a prime mover, for example, a diesel engine, gas engine, gas turbine, etc., and/or to reduce harmful components such as NO.sub.X (nitrogen oxides) in the exhaust gas.
2. Description of the Related Art
Prior art arrangements concerning cogeneration denitration systems for use in prime movers for generators may be roughly divided into three types which are shown in FIGS. 4, 5 and 6, respectively. The cogeneration system shown in FIG. 4 is applied to uses where the temperature of exhaust gas emitted from a prime mover a (in the case of a diesel engine) is about 450.degree. or less. In this system, a denitration apparatus b is connected to an exhaust pipe line l.sub.1 of prime mover a for driving a generator. The denitration apparatus b is injected with ammonia (NH.sub.3) to reduce and thereby minimize NO.sub.X (nitrogen oxides) in exhaust gas in the presence of a catalyst. The high-temperature exhaust gas which has been treated in the denitration apparatus b is supplied through an exhaust pipe line l.sub.2 to an exhaust heat exchanger c where heat is recovered from the exhaust gas, and the treated gas is then released into the atmosphere through an outlet piping l.sub.3 and a stack d.
The cogeneration system shown in FIG. 5 is applied to uses where the temperature of exhaust gas from a prime mover a is about 500.degree. C. as in the case of a small-sized, high-speed diesel engine. In contrast to the system shown in FIG. 4, this system is arranged such that an exhaust heat exchanger c is connected to the exhaust pipe line l.sub.1 of the prime mover a, and a denitration apparatus b is connected to the exhaust pipe line l.sub.2 of the exhaust heat exchanger c, whereby exhaust gas emitted from the prime mover a is first supplied to the exhaust heat exchanger c where the gas is cooled by heat exchange and the resulting low-temperature exhaust gas is then supplied to the denitration apparatus b so that the exhaust gas is maintained at a temperature below the higher limit of the range of exhaust gas temperatures applicable to the catalyst incorporated in the denitration apparatus b.
In contrast to the system shown in FIG. 5, the cogeneration system shown in FIG. 6 is arranged such that a second exhaust heat exchanger c is additionally connected to the exhaust pipe line l.sub.3 of a denitration apparatus b, whereby the exhaust gas after the heat recover carried out in a first exhaust heat exchanger c which is disposed at the upstream side of the denitration apparatus b is further subjected to heat recovery in the downstream-side exhaust heat exchanger c, thereby further lowering the exhaust gas temperature.
Each of the above-described denitration apparatuses b is arranged such that NO.sub.X in the exhaust gas is reduced by ammonia injected in the presence of a catalyst. The operating temperature of the catalyst is given as a function of the SO.sub.3 concentration with a view to preventing deterioration of the catalyst performance and therefore restricted within a predetermined temperature range. The catalyst performs the following reducing reactions to decrease NO.sub.X (nitrogen oxides): EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N+6H.sub.2 O EQU 2NO.sub.2 +9NH.sub.3 +4O.sub.2 .fwdarw.5N.sub.2 +12H.sub.2 O
Each of the above-described exhaust heat exchangers c is arranged in the form of a heat recovery mechanism wherein exhaust gas is cooled by heat exchange with feed water (coolant), while the feed water is heated by such heat exchange and applied to various uses in the form of hot water or steam. In FIGS. 4-6, the reference symbol a.sub.1 denotes an air supply pipe, a.sub.2 a fuel supply pipe, c.sub.1 a feed water (coolant) inlet pipe, and c.sub.2 a feed water (coolant) outlet pipe.
The above-described conventional cogeneration denitration systems suffer, however, from the following problems.
The cogeneration system shown in FIG. 4 is not applicable to the case where the temperature of exhaust gas emitted from a prime mover exceeds about 450.degree. C., that is, the higher limit of the applicable temperature range of the catalyst incorporated in the denitration apparatus. In the cogeneration system shown in FIG. 5, lowering of the exhaust gas temperature in the exhaust heat exchanger is restricted by the lower limit of the applicable temperature range of the catalyst in the denitration apparatus, so that it is impossible to satisfactorily lower the exhaust gas temperature in the exhaust heat exchanger and it is therefore necessary to by-pass the exhaust gas, resulting in low heat recovery efficiently. Other exhaust gas purifying catalysts, for example, a desulfurizing catalyst and the like, also have respective applicable temperature ranges and therefore suffer from the same problems as in the the case of the above-described denitration catalyst.
In the cogeneration system shown in FIG. 6, two exhaust heat exchangers are provided at the upstream and downstream sides, respectively, of the denitration apparatus, and therefore the system suffers from the following problems: the mechanism is complicated by a large margin; the overall size of the system is increased; the production cost is high; and it is difficult to provide sufficient installation space therefor.